This invention relates to a detection system and, in particular, to an acoustic reflectometer sensor for detecting the presence of objects such as a paper in a paper path.
In machines requiring the movement of sheets of paper in timed sequence, such as printers and reproduction machines, paper jamming often occurs due to improper paper feeding, spacing inaccuracies, and various other factors resulting in the improper deceleration or acceleration of paper sheet speeds in the machine.
The prior art is replete with paper sensing devices to sense the presence of sheets of paper at various points along the path of travel. One type of sensing device takes the form of switches actuated by switch arms located in the path of movement of the sheet. The disadvantages of this type of sensor are the response time of the control to the mechanical actuation of the switch by the paper. Also, the fact that the sheet of paper must contact a switch arm itself may effect the travel of the sheet, either retarding the advancement of the sheet or skewing the sheet out of the predetermined path.
Other sheet detection systems utilize photodetectors combined with light sources for sensing sheet presence or absence. One disadvantage of this type of sensor is the accumulation of dust and other material decreasing the sensitivity of the device. Another disadvantage is that the photodetector, light source pair will not in general sense transparent or translucent sheets of paper. Also, in particular in photocopy machines, it is often necessary to provide light baffles or other enclosures to insure that the machine photosensitive surface does not come into light contact with the light sources of the sensors.
Other types of detection devices such as disclosed in U.S. Pat. No. 3,603,680 teach the use of a plurality of ultrasonic detecting devices dispersed along the path. Each of the detecting devices includes an ultrasonic transmitting transducer for generating ultrasonic waves of a predetermined wavelength and an ultrasonic receiving transducer to receive the transmitted waves. The acoustically vibrating element in each of the transducers is generally a piezoelectric material for converting electrical signals to mechanical vibrations or mechanical vibrations to electrical signals. The detection devices are arranged along the paper path and circuit means monitor in timed sequence the effect upon each of the detection devices as sheets are transported.
Other sensors such as disclosed in U.S. Pat. No. 3,479,026 teach the use of a hollow tube attached to a speaker. In operation, the speaker is driven at constant frequency and the acoustical impedance of the hollow tube is measured by measuring the electrical impedance of the speaker. When a document moves along the paper path and covers one end of the hollow tube, the acoustical impedance and thus the electrical impedance is changed. A difficulty with this type of system and other prior art systems is that it is necessary to provide a separate transmitter and/or receiver along each port or location of the paper path where it is desired to sense the presence or absence of a sheet of paper. This can be relatively complicated and expensive.
It would be desirable, therefore, to provide a paper sensing system that is simple and relatively inexpensive and provides the means to detect the presence or absence of paper at several ports along a paper path without the need to provide a separate transmitter and receiver at each port.